In general, positive-type photoresist compositions which have been used are those containing an alkali soluble resin and a naphthoquinonediazide compound functioning as a light-sensitive material. For instance, they are described, e.g., in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470 as "novolak type phenol resin/substituted compound of naphthoquinonediazide". As the most typical examples of such compositions, L.F. Thompson, Introduction to Microlithography, No. 219, pp. 112-121, ACS Publishers describes examples of "cresolformaldehyde type novolak resin/trihydroxybenzophenone-1,1-naphthoquinonediazidosulfonic acid ester".
Novolak resins are soluble in an alkaline aqueous solution without swelling therein, and confer high resistance to etching, especially plasma etching, upon produced images when the images are used the etching mask. This is because they are particularly effective as binders. On the other hand, naphthoquinonediazide compounds used as a light-sensitive material are peculiar in the respect that though they themselves act as dissolution inhibitor which lowers the solubilities of novolak resins in an alkali, they produce an alkali-soluble substance when irradiated with light to undergo decomposition, resulting in rather heightening the solubilities of novolak resins in alkali. Because of a great change in their nature upon optical exposure, naphthoquinone diazide compounds are particularly useful as a light-sensitive material.
From those standpoints, a number of positively working photoresists comprising a novolak resin and a naphthoquinonediazide type light-sensitive material has been developed, and practically used. Although they have accomplished satisfactory results in line width processing of the order of 1.5 to 2 microns, the integrated circuits at present come to have an increasingly raised degree of integration, and require the processing of hyperfine patterns with a line width of below 1 micron in the production of semiconductor substrates for VLSI (very large-scale integrated circuits) and the like. For such use, it is required of photoresists to be endowed with a particularly high resolution, a high accuracy in pattern-shape reproduction or a great ability to duplicate accurately the shape of a exposure mask, and a high sensitivity from the standpoint of high productivity. That is, it is the real situation that conventional positive-type photoresists as described above cannot answer to the needs.